The effect of SiO₂ nanoparticles derived from hydrothermal solutions on the performance of portland cement based materials

Abstract

The nanoparticles of SiO₂ were used in cement systems to modify the rheological behavior, to enhance the reactivity of supplementary cementitious materials, and also to improve the strength and durability. In this research, low-cost nano-SiO₂ particles from natural hydrothermal solutions obtained by membrane ultrafiltration and, optionally, by cryochemical vacuum sublimation drying, were evaluated in portland cement based systems. ƒThe SiO₂-rich solutions were obtained from the wells of Mutnovsky geothermal power station (Far East of Russia). The constant nano-SiO₂ dosage of 0.25% (as a solid material by weight of cementitious materials) was used to compare the cement systems with different nanoparticles against a reference mortar and a commercially available nano-SiO₂. Nanoparticles were characterized by X-Ray Diffraction (XRD), BET Surface Area, Scanning Electron Microscope (SEM) and Fourier Transform Infrared (FTIR) spectroscopy techniques. It was demonstrated that the addition of polycarboxylate ether superplasticizer and the dispersion treatment using an ultrasound processor can be used to facilitate the distribution of nano-SiO₂ particles in the mixing water. The effect of nano-SiO₂ particles in portland cement mortars was investigated by evaluating the flow, heat of hydration and compressive strength development. It was demonstrated that the use of nano-SiO₂ particles can reduce the segregation and improve strength properties.

Keywords

ultrafiltration / cryochemical vacuum sublimation drying / nanoparticles / portland cement / heat of hydration / surface area / compressive strength

Introduction

There is an ongoing quest to improve the properties of concrete which can be achieved at different scale levels (Fig. 1). At a macroscale, the optimization of aggregates is used to reduce the consumption of cementitious materials [1–6] and also to improve the performance of concrete mixtures [7]. The addition of air entraining admixtures and engineering of specific air-void structure are commonly used to enhance the resistance of concrete against freeze-thaw damage. At a sub-micro- and micro- scale, the addition of supplementary cementitious materials was used to improve the long-term mechanical response and durability of cement based materials [8,9]. At nanoscale, nanoparticles of SiO₂ were used in cement composites to modify the rheological behavior, to enhance the reactivity of supplementary cementitious materials, as well as to improve the strength and durability [4,5].

The nanoparticles of SiO₂ were extensively used to increase the strength and durability of concrete [1–3,10]. Björnström et al. investigated the effects of colloidal silica on the hydration process of Portland cement minerals C₃S (alite, Ca₃SiO₅) [10] and b-C₂S (belite, b-Ca₂SiO₄) [1]. It was demonstrated that nanoparticles accelerate the formation of C-S-H gel within the first 24 hours, especially when used at higher dosages of up to 2%. Collepardi et al. [2] produced self-consolidating concrete with low-heat of hydration using up to 2% of colloidal silica as a viscosity modifying and pozzolanic agent. It was observed that nanoparticles enable the production of cohesive concrete with reduced segregation and bleeding. Ye et al. investigated the development of Ca(OH)₂ phase in pastes and concrete with silica fume and nanosilica [3]. The nanosilica increased the viscosity and reduced the setting time of pastes. In concrete, with addition of nano-SiO₂ the quantity and the size of Ca(OH)₂ phase crystals formed in the interfacial transition zone were reduced and the mechanical performance at early ages was better than for specimens produced with silica fume. Flores-Vivian et al. [4] studied the effects of sol-gel synthesized nanosilica on strength of portland cement mortars. The addition of superplasticizer and an effective dispersion were employed to distribute these nano materials within the cementitious matrix and enhance the mechanical performance of mortars. Gaitero et al. [11] demonstrated that the addition of nano-SiO₂ to cementitious systems can improve the strength by controlling the structure of C-S-H. Nano-SiO₂ with particle size between 5 and 50 nm was used in the field project as a viscosity modifying agent at a dosage of 1-2% of cementitious materials [2,12,13]. Despite of attractive performance, the application of nanoparticles in concrete remains limited because of high cost associated with the production of nanomaterials.

However, low cost silica nanoparticles can be obtained from natural sources such as hydrothermal solutions formed due to magmatic ore intrusion. When magma cannot come to the surface, because of resistance of rocks above the magmatic chamber, the hydrothermal activity develops around the chamber. Steam or hot water erupting from the earth surface due to geothermal activities has been used by Romans, Chinese and Native Americans for bathing and processing of food [14]. Currently, high-temperature steam and water are used for power generation and also as a supply of heat. In a geothermal power plant (Fig. 2), hot water rises to the surface, evaporates, and the steam is redirected to the turbines and a generator that produces energy. The steam is condensed into water in a cooling tower, where minerals (including nanosilica) can be recovered and utilized for industrial applications[15]. The minerals found in geothermal power plant water streams include zinc, silica, lithium, manganese, boron, lead, silver, antimony and strontium [16]. In geothermal plants, silica-particles cause clogging of pipes and, therefore, nanosilica extraction increases the efficiency of the plant [14]. Nanosilica present at high concentrations in hydrothermal solutions can be extracted for subsequent application in various products such as paint, paper, toothpaste, tires, chocolate slim shakes, solar panels [16] and also as a pozzolanic material for use in concrete [6,15].

In volcanic areas, orthosilicic acid is formed in hydrothermal solutions due to the dissolution of aluminosilicate minerals of rocks under elevated pressures and temperatures. Concentrated solutions travel to the surface, where the pressure and temperature is lower, and due to hydrolysis and polycondensation, spherical silica nanoparticles are formed [17,18]. In this research, SiO₂ particles were obtained through membrane ultrafiltration and tested in portland cement mortars. The hydrothermal solutions were collected from the wells of the Mutnovsky geothermal power station (Far East, Russia). The use of hydrothermal solutions as a relatively cheap natural precursor for nano-SiO₂ production can reduce the costs associated with the production and application of nanomaterials [17,18].

Material and methods

Natural silica nanoparticles

Silica nanoparticles were obtained from the hydrothermal solutions in the form of silicic acid (H₄SiO₄). The initial solution also contained other components listed in Table 1. Nanodispersed powders were obtained from the solutions by removing the excess water by filtering through membrane devices and, for selected specimens, by a cryochemical vacuum sublimation drying (e.g., with the use of liquid nitrogen) [18,19]. For membrane filtering, polyether-sulfone and poly (acrylonitrile) capillary ultrafiltration membranes with pore sizes from 20 to 100 nm and tubular ceramic microfiltration membranes with an average pore diameter of 70 nm were used. For sublimation drying, slightly aggregated nanodispersed powders with size of 30–50 microns were obtained from concentrated aqueous sols. The sol drops were passing to the chamber with liquid nitrogen at temperature of-196°C and about atmospheric pressure. After drops cryocrystalized to the solid state the pressure was 3–8 Pa and temperature changed during the process from –50°C to 20°C at the lowest level. Commercially available nano-SiO₂ admixture Cembinder-8 (CB8, available in a form of 51.5% water suspension) from Eka Chemicals was used as a reference nano-SiO₂ product.

Three types of nanoparticles were produced by ultrafiltration from the geothermal solutions in the form of a gel (specimen MB) and two dry powder specimens (TB and N2) by subsequent sublimation drying using liquid nitrogen. The temperature used for the formation of TB particles was higher than that used for N2 and so larger TB particles were characterized by reduced surface area.

Dry silica specimens were pulverized in a ceramic mortar with the addition of acetone. The test for BET specific surface area included three steps: sample preparation, degasification, and analysis. For each step, the weight of the sample was controlled. A maximum temperature of 200 °C and vacuuming for 2 hours were used for degasification. This procedure was used to remove any moisture or solvent residues from the samples. The XRD powder diffraction and scanning electron microscope techniques were used to analyze the silica powders. The results of the BET investigation are reported in Table 2. In Fig. 3, an X-ray diffractogram (left) and SEM image (right) reveals an amorphous structure of spherical silica nanoparticles.

Cementitious materials and admixtures

Portland cement (PC) conforming to ASTM Type I was supplied by Lafarge. The chemical composition and physical properties of the cement are presented in Table 3, along with the requirements of ASTM Standard Specification for Portland Cement (ASTM C150). Commercially available polycarboxylate superplasticizer (PCE/SP, Megapol GUSR-AC with 38.7% solid concentration supplied by Handy Chemicals) was used as a surfactant. Graded Ottawa sand (ASTM C778) was used as a fine aggregate for preparation of mortars [20]. Deionized water was used for the preparation of mortars.

Processing of silica nanoparticles

The dispersion of nanoparticles with PCE superplasticizer was achieved by an ultrasound processor (Hielscher UIP1000hd). The PCE was first mixed with water using a glass stick until the uniform dissolution was observed. The solution was further treated using the ultrasound (at 20 kHz) processor at 50% of the maximum power (750 W) for 30 seconds to ensure the effective dispersion. The nanoparticles were added to PCE solution and dispersed using the ultrasound processor at 75% of the maximum power for 6 minutes. A water bath filled with ice and cool water was used to control the temperature (<30 ºC) during the process.

Characterization of silica nanoparticles

Fourier Transform Infrared (FTIR) spectroscopy was used to compare the molecular structure of silica nanoparticles as reported in Fig. 4. The bonds corresponding to water molecules were observed in CB8 and MB samples at the wave length of 900 to 980 cm⁻¹. Stretching vibration characteristic for Si-(OH) bonds at 900 to 980 cm⁻¹ and Si-O-Si bonds at approximately 800 cm⁻¹ and 1000 to 1250 cm⁻¹ were observed for investigated samples.

Different types of Si-groups on the surface and in the bulk of the silica grains are represented by Fig. 4(b) [6,8]. These can be surface free isolated silanol groups ≡SiOH (Q³ type); surface free geminal (isolated) silanol or silanediol groups=Si(OH)₂ (Q² type); vicinal bridging silanol groups, single geminal groups, i.e., hydrogen-bonded surface single silanol groups, single geminal groups, and their combinations; ≡Si–O–Si≡ siloxane bridges with the O atom on the surface (Q⁴ type); and internal silanol groups located within the skeleton and (or) in very thin ultramicropores of silica. Therefore, unmodified surface of amorphous silica can contain only two main types of OH groups; single and geminal groups, which in turn, are subdivided into isolated, free, and hydrogen-bonded vicinal silanol groups [21].

The intensity of the stretching vibration peak corresponding to the Si-O-Si bond can be correlated to the number of bonds present in the material. The lack of Si-(OH) bonds in the CB8 specimen is a sign of very poor coordination between the silica and water (H-O-H) molecules (or OH^- groups). A higher crystallinity of SiO₂, or the presence of a surfactant (stabilizing agent) attached to the silica particles, may be the reason for the absence of Si-(OH) bonds in the CB8 sample. In this way, higher intensity corresponding to the Si-O-Si bond and the absence of Si-(OH) bonds can suggest that CB8 is a more ordered material. Therefore, more ordered molecular structure of CB8 can be expected (Fig. 4). This can reduce the reactivity of CB8 in portland cement systems; however, due to very small particle size a seeding (nucleation) effect may still be very pronounced.

Lower intensity corresponding to the Si-O-Si bond observed at 1000 to 1250 cm⁻¹ for nanosilica harvested from hydrothermal solutions suggests the formation of poorly ordered structures (Fig. 4). The intensity of Si-O-Si bonds can be reduced due to the development of Si-(OH) bonds on synthesized nanoparticles. Lower intensity of the Si-O-Si bonds and the presence Si-(OH) bonds suggest that all hydrothermal silica nanoparticles are highly amorphous materials. This can potentially improve the reactivity of nanoslica in portland cement systems, indicating a high capacity for seeding (nucleation) and pozzolanic effects. Here, the FTIR study confirms the results of the XRD investigation.

Testing procedures for mortars

For preparation of reference mortar specimens, the PCE superplasticizing admixture was dissolved in a portion of mixing water, and then the nanoparticles of CB8 (when required) were added and stirred for 3 minutes at a high speed. The mixing process for mortars with developed nanoparticles included the addition and mixing of cement and aggregates for one minute in the mixer and then the addition of two thirds of the water containing the pre-dissolved nanoparticles and subsequent mixing for 3 minutes. Then, the remaining water was introduced (with a portion of PCE superplasticizer if required) and the mix was blended for another two minutes. Therefore, the whole mixing process took six minutes to complete.

The constant nano-SiO₂ dosage of 0.25% (as a solid content by weight of cementitious materials) was used to compare all nanoparticles against a reference mortar without nanoparticles. Water-to-cement ratio (W/C) of 0.3, sand-to-cement ratio (S/C) of 1.0 and a PCE dosage of 0.15% (by weight of cement) were used to produce mortar mixtures.

Portland cement hydration is an exothermal reaction affected by the addition of chemical admixtures and supplementary cementitious materials. The isothermal conduction calorimeter (TAM Air from TA Instruments) was used to monitor the hydration process for 72 hours at 21±1 °C. The output of the calorimeter was used to evaluate the performance related to setting, early strength development and the effect of nano-SiO₂. The initial setting time (IST) was calculated when the slope of the isothermal curve increased from zero to a positive value. The final setting time (FST) was calculated as the time required to reach 50% of the average maximum power corresponding to the main hydration peak as described in [8,22–25]. The total hydration energy (THE) produced during the hydration was calculated by measuring the area under the 24-hour isothermal curves starting from the IST and until the isothermal curves became flat. Relevant standards were used for the evaluation of the heat of hydration (ASTM C1679, 2009) [23,25], flow of mortars (ASTM C1437, 2007) [26], compressive strength (ASTM C109, 2007) [27] and splitting tensile strength (IS 5816 adopted for 50 mm cube testing) [25–30]. The reported compressive strength and splitting tensile strength values are average of three and two specimens, respectively.

Results and discussion

Properties of fresh mortars

The flow of mortars was determined for all mixtures and reported in Table 4. The reference material (Ref) had the highest flow among all tested mortars. The addition of well dispersed nano-SiO₂ at low dosage (0.25%) had a significant impact on the rheological response of the mortars. Commercially available nano-SiO₂ (CB8) which is commonly used as a viscosity modifying agent had the lowest surface area (Table 2). However, the nanoparticles of CB8 are well dispersed and demand higher volumes of water to achieve the required levels of workability. Therefore, at the same W/C, the addition of CB8 reduces the flow of mortar.

Powder TB particles had the least impact on the flow, most probably due to the lowest surface area. The N2 had the highest flow reduction compared with the rest of the nanosilica products. The high surface area and a higher degree of agglomeration as well as insufficient dispersion may be the causes for the flow reduction in N2 based mortars. A moderate effect was observed in the case of nanosilica sol (MB) material. It was expected that MB would have the highest impact on the flow due to its highest surface area.

The research results indicate that the addition of nano-SiO₂ increases the cohesiveness and reduce the bleeding and segregation in portland cement based systems. The source type, the impurities, crystallinity and agglomeration of nanosilica particles are the possible causes affecting the rheological properties and flow of mortars. Therefore, to keep the specified workability levels in the systems with nano-SiO₂, the dosage of superplasticizer can be increased.

Hydration of mortars

The effects of hydrothermal nanosilica products such as TB, N2 and MB on the hydration process of mortars was analyzed by monitoring the heat of hydration with an isothermal calorimeter using a reference mix for comparison (Fig. 5). The isothermal calorimetry responses were used to determine the setting times and heat released during the hydration (Table 4).

The first (C₃S) and second (C₃A) peaks for the reference system and mortars with CB8 were observed at 10.5 and 14 hours after mixing with C₃A characterized by a lower heat flow.

The addition of powder nano-SiO₂ products (such as N2 and TB) had a considerable effect on the heat of hydration. The N2 nanoparticles had a profound increase of heat of hydration (HOH). The increased rates of C₃S hydration can be correlated with the BET surface area, as higher surface area (SSA) must lead to the acceleration of C-S-H gel formation due to the seed effect. With the addition N2 particles, the first peak corresponding to C₃S hydration was observed after 10 hours followed by second peak at 14 hours (from mixing with water) (Fig. 6). For this composition, the rate of hydration heat was about 5% higher compared with the reference and CB8 based mortars (Table 4).

For TB mortars, the first peak (C₃S) was observed after 11.3 hours and the second peak (corresponding to C₃A) was identified at 15 hours at the rate of hydration of 5% lower compared with the reference and CB8 based mortars. With the addition of nano-SiO₂ the hydration of C₃S was accelerated due to a heterogeneous nucleation effect. Heterogeneous nucleation is typically the dominant acceleration mechanism when well dispersed seed component with a size smaller than cement particles is incorporated.

The extension of initial setting time was due to the addition of TB and N2 as reported in Fig. 7. However, the final setting time for TB and N2 mortars was shortened by 36 minutes and 54 minutes, respectively compared with the reference. This effect was expected as nanoparticles with higher specific area accelerate the cement hydration process and reduce the setting time. Mortars with powder nano-SiO₂ (e.g., N2 with the highest SSA) were characterized by the highest heat of hydration followed by CB8 and TB products. In addition to pozzolanic activity, nano-SiO₂ particles with higher SSA increases the wettable surface area and so provide the bulk of the nucleation sites required for the formation of hydration products at very early stages. However, for MB, the sol based nano-SiO₂ product, the hydration of C₃S was hindered and C₃A hydration was accelerated. In addition to this, the MB additive provided a shorter setting time compared with the reference.

The C-S-H is the main product of cement hydration and a primary binding phase in portland cement. When superfine particles with higher surface area (such as nanoparticles) are added, a denser matrix is created due to the formation of uniform, well distributed C-S-H gel structure. As a result, the mechanical performance and durability in cement systems with nanosilica is improved [29,30]. The increased hydration rates of C₃S can be correlated to BET surface area as the materials with higher surface area accelerate the formation of C-S-H gel (Fig. 7).

Mechanical performance

The compressive and splitting tensile strengths of mortars with nano-SiO₂ at different curing ages are reported in Fig. 8. At the age of 28 days the specimens containing MB nano-SiO₂ had the highest compressive strength of 110 MPa. There was a slight increase of 1-day strength in mortars with MB and N2, which corresponds to about 12% and 19% improvement compared with the reference.

The splitting tensile strength of investigated mortars followed similar trends and can be correlated to compressive strength, especially at the age of 3 days and afterwards (Fig. 9). It can be observed that the use of some nano-SiO₂ products obtained from hydrothermal solutions at a very small dosage of 0.25% (by weight of cementitious materials) can provide considerable strength enhancement for mortars.

Conclusions

Based on the results of reported investigation, it can be concluded that the use of nano-SiO₂ particles obtained from the hydrothermal solutions can improve the performance of portland cement.

As detected by FTIR, the powder nano-SiO₂ products had a lower intensity of Si-O-Si bonds compared with nano-SiO₂ sol products. The degree of disorder (as opposite to crystallinity) of nano-SiO₂ structure can play a role in the reactivity and strength development of mortars with nanomaterials.

The hydration of portland cement systems can be accelerated by nano-SiO₂ due to higher surface area and adequate dispersion of nanoparticles. The C₃S hydration rate can be correlated with the BET surface area of nano-SiO₂ particles, as the higher surface area accelerated the formation of C-S-H gel due to seeding effect.

Typically, finer particles with higher surface area react more vigorously than the larger particles. With the addition of nano-SiO₂ the setting time is shortened due to accelerated hydration.

The powder nano-SiO₂ products with higher surface area can accelerate the hydration of cement and provide enhancement of early-age strength. Due to ongoing pozzolanic reactions, nano-SiO₂ additives can modify the structure and morphology of C-S-H products resulting in denser structure and improved mechanical performance of mortars. The addition of very small dosage of nano-SiO₂ such as 0.25% by the weight of cementitious materials combined with 0.15% of PCE superplasticizer (partially used for the dispersion of nano-SiO₂) can provide consistent, up to 10% improvement of mortar strength in all ages of hardening.

References

[1] Björnström J, Panas I. Antagonistic effect of superplasticizer and colloidal nano-silica in the hydration of alite and belite pastes. Journal of Materials Science, 2007, 11(42): 3901–3907

[2] Collepardi M, Ogoumah J, Skarp U, Troli R. Influence of Amorphous Colloidal Silica on the Properties of Self-Compacting Concretes. In: Proceedings of the International Conference, Challenges in Concrete Construction- Innovations and Developments  in  Concrete  Materials  and  Construction, Dundee, UK, 2002.

[3] Ye Q, Zhang Z, Kong D, Chen R. Influence of nano-SiO2 addition on properties of hardened cement paste as compared with silica fume. Construction & Building Materials, 2007, 21(3): 539–545

[4] Flores-Vivian I, Sobolev K, Torres-Martinez L, Cuellar E, Valdez P, Zarazua E. Performance of Cement Systems with Nano- SiO2 Particles Produced Using Sol-gel Method. Transportation Research Record, 2010, 1: 10–14

[5] SobolevK. Modern developments related to nanotechnology and nanoengineering of concrete.Frontiers of Structural and Civil Engineering, 2016, 10(2): 131–141

[6] PotapovV, ShitikovE, TrutnevN, GorbachV, PortnyaginN. Influence of Silica Nanoparticles on the Strength Characteristics of Cement Samples.Glass Physics and Chemistry, 2001, 1(37): 98–105

[7] MoiniM, Flores-VivianI, AmirjanovA, SobolevK. The optimization of aggregate blends for sustainable low cement concrete.Construction &Building Materials, 2015, 93: 627–634

[8] Langan B, Weng K, Ward M. Effect of silica fume and fly ash on heat of hydration of portland cement. Cement and Concrete Research, 2002, 32(7): 1045–1051

[9] Flores-Vivian I, Pradoto R, Moini M, Sobolev K. The use of nanoparticles to improve the performance of concrete. In: Nano Conference, Brno, Czech Republic, EU, 2013

[10] Björnström J, Martinelli A, Matic A, Börjesson L, Panas I. Accelerating effects of colloidal nano-silica for beneficial calcium–silicate–hydrate formation in cement. Chemical Physics Letters, 2004, 392(1–3): 242–248

[11] Gaitero J J, Campillo I, Guerrero A. Reduction of the calcium leaching rate of cement paste by addition of silica nanoparticles. Cement and Concrete Research, 2008, 38(8–9): 1112–1118

[12] Jansson I, Skarp U, Bigley C. The value of colloidal silica for enhanced durability in high fluidity cement based mixes. In: the 5 International RILEM symposium on self-compacting concrete, 2007

[13] Green B. Development of a high-density cementitious rock-matching grout using nano-particles. In: Proceedings of ACI Session on “Nanotechnology of Concrete: Recent Developments and Future Perspectives”, 2008

[14] GawellK. Can Geothermal Energy Offset Global Warming? Renewable Energy News & Information, 2008

[15] SobolevK. Nanotechnology and Nanoengineering of Construction Materials.In: Nanotechnology in Construction, Proceedings of NICOM5, 2015, 3–13

[16] KagelA. The State of Geothermal Technology Part II: Surface Technology.Geothermal Energy Association, 2008

[17] Yokogawa Corporation of America.Yokogawa in the Power Industry,Bulletin 53T01A01-01E, 2005

[18] KutepovA, PotapovV. Movement and mass exchange of liquid drop in spinned flow of geothermal medium.Tear. Osnovy Khim. Tekh., 2000, 34(2)

[19] Brazhnikov S, Generalov M, Taitnev N. Vacuum Sublimation Technique for Preparing Ultradispersed Powders of Inorganic Salts. Khim. Mashinoslr. (Moscow), 2004, 12: 12–15

[20] ASTM C778. American Society for Testing and Materials. Standard specification of standard sand, 2006, 372–374

[21] Zhuravlev L. The surface chemistry of amorphous silica. Zhuravlev model. Colloids and Surfaces. A, Physicochemical and Engineering Aspects, 2000, 173(1): 1–38

[22] Mindess S, Young J F, Darwin D. Concrete, 2nd ed., Upper Saddle River, NJ: Prentice Hall, 2003

[23] ASTM C1679 – 09. American Society for Testing and Materials. Standard Practice for Measuring Hydration Kinetics of Hydraulic Cementitious Mixtures Using Isothermal Calorimetry, 2009

[24] Wang K, Ge Z, Grove J, Ruiz J M, Rasmussen R, Ferragut T. Developing a Simple and Rapid Test for Monitoring the Heat Evolution of Concrete Mixtures for Both Laboratory and Field Applications. Center for Transportation Research and Education, Iowa State University, 2007

[25] Muzenski S, Flores-Vivian I, Sobolev K. Hydrophobic engineered cementitious composites for highway applications. Cement and Concrete Composites, 2015, 57: 68–74

[26] ASTM C1437-07. American Society for Testing and Materials. Test Method for Flow of Hydraulic Cement Mortar, 2007, 611–612

[27] ASTM C109-07. American Society for Testing and Materials. Compressive Strength of Hydraulic Cement Mortars (using 2-in or 50-mm Cube Specimens), 2007, 64–68

[28] IS 5816:1999. Splitting tensile strength of concrete- Method of Test, Bureau of Indian standards, 1999

[29] Quercia G, Spiesz P, Husken G, Brouwers J. Effects of amorphous nano-silica additions on mechanical durability performance of SCC mixtures. In: International Congress on Durability of Concrete, Trondheim, Norway, 2012

[30] Sobolev K, Lin Z, Flores-Vivian I, Pradoto R. Nano-Engineered Cements with Enhanced Mechanical Performance. Journal of the American Ceramic Society, 2016, 99(2): 564–572

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